Why do we say that someone with a hidden agenda has “an axe to grind”?

As a boy, Benjamin Franklin was sharpening tools in his father’s yard
when a stranger carrying an axe came by and praised the boy on how
good he was with the grindstone. He then asked Franklin if he would
show him how it would work on his own axe. Once his axe was sharp-
ened, the stranger simply laughed and walked away, giving young
Franklin a valuable lesson about people with “an axe to grind.”